Soot sensors, also known as particulate matter (PM) sensors, are often used in vehicles having diesel engines. A particulate matter sensor may be located upstream from a particulate filter, where the sensor is exposed to exhaust flow from the engine having soot particles entrained in the exhaust gas. Alternatively, a particulate matter sensor may be located in a gas stream downstream from a particulate filter, where the sensor is used to monitor the proper operation of the particulate filter.
A known method of sensing soot uses a particulate matter sensor having two electrodes that are spaced from one another and exposed to the gas stream. In the absence of soot, there is very low electrical conductivity (high electrical resistance) between the electrodes. As soot accumulates on the surface of the sensor, soot particles act to bridge the gap between the electrodes. Because the soot particles are electrically conductive the conductivity between the electrodes increases, and this change in conductivity can be related to the amount of soot in the gas stream. Sensors that operate according to this principle are disclosed in US Patent Application Publication 2008/0283398, US Patent Application Publication 2008/0282769, and U.S. Pat. No. 7,954,230, the contents of each of which are hereby incorporated by reference in their entirety.
While the PM sensor is intended to indicate the presence of soot in the exhaust stream, it is possible that a contaminant other than soot may be deposited on the surface of the PM sensor. Such a contaminant may comprise material from additives in fuel or lubricating oil that enter the gas stream to which the sensor is exposed. The presence of such a contaminant on the surface of the particulate matter sensor may interfere with the ability of the sensing electrodes to receive the conductive particulate matter that the sensor is intended to detect, thus preventing the sensor from properly indicating the presence and/or amount of soot in an exhaust stream.
Government regulations require that the particulate matter sensor have self-diagnostics (i.e. On Board Diagnostics or OBD) capability to verify that it is functioning properly. The sensor must be able to verify that the circuit is functioning properly and that if soot lands on the electrode, the sensor can detect it. However, in the presence of contaminants on the sensor, this can be difficult.
Accordingly, the inventors herein have recognized a need for an improved sensing system having a particulate matter sensor that reduces and/or eliminates the foregoing deficiencies.